You're not doing anything wrong, you're just dealing with a growth spurt. I'm going to make a wild guess that you're somewhere between 11 and 14 years old, that a lot of skills that used to be easy now feel harder, and even the ones you can still do feel a little off, a little inconsistent, a little unsteady.
The good news: Just by continuing to do gymnastics, it will fix itself over time without requiring any particular conscious effort from you.....
The bad news: .... but it will probably take six months to a year.
Not much you can do except be patient and stick with it. In general, doing more core strength might help a little bit; as your arms and legs get longer, your core as to work comparatively harder to keep everything stable while they move. Things like hollow rockers, arch rockers, side rockers, planks, etc.
Also this is a good time to focus on basics. Most of the troubles you'll have with upper-level skills can be traced to basics that tend to deteriorate during a growth spurt.
But really, it just takes time. On a micro-level, here's what's going on:
Generally when you're training gymnastics, you are consciously focusing on big picture technique which is pretty much the same for everybody. But subconsciously, you're also learning all sorts of microadjustments and very precise angles and a very precise balance of exertion between different muscles on any given skill, and all of these microadjustments are unique to your body. No two bodies are identical, and so there are always slight differences in where the weight is distributed, and how strong each muscle is relative to your weight and the lengths of your arms and legs, and so on and so forth, and that's all stuff you learn subconsciously by feel.
To give a more concrete example of this: consider a cast to handstand: every athlete has to lean her shoulders in front of the bar and then extend the shoulders with the correct speed and timing so that everything balances right over the bar as it aligns at vertical. However, the exact perfect angle to lean, the exact perfect speed at which to extend the shoulders, the exact amount each muscle needs to activate, all of this is different from athlete to athlete, and can't really be coached or trained, other than just by doing casts and developing a feel for it.
Over the years you've spent training, your brain has -- completely without any conscious effort by you -- put together an encyclopedia of precise angles and timings and ways to adjust when those angles or timings are off and muscle memory that is specific to the exact strength of your muscles and your exact weight distribution and the exact lengths of your limbs and so on....
.... and then your arms and legs got longer, and that entire subconscious encyclopedia has to be thrown out and rebuilt. It's frustrating, and it's easy to see why a lot of athletes quit at this age (and if that's what you decide to do, that's fine, there's nothing wrong with that). BUT if you're patient enough to stick it out, most* of the skills will come back.
*The flexibility might or might not come back, but if it doesn't there are always workarounds
